The present invention relates to a cam mechanism for a lens barrel and more particularly to a cam mechanism for a lens barrel, wherein the cam mechanism prevents the slant of a movable annular member set in a cam ring.
Generally, a lens barrel is provided with various types of cam mechanisms for such purposes as zooming and focusing. This cam mechanism basically includes a cam ring with a cam groove formed therein, and a movable annular member (for example, a lens frame) set inside the inner circumference or outer circumference of this cam ring in such a manner as to permit its free rotating movement. The cam mechanism further includes a cam pin fitted into the cam groove formed in the cam ring. In a conventional lens barrel, which is constructed in this manner, the movable annular member will be moved in accordance with the profile of the cam groove, for example, by the rotation of the cam ring.
In this regard, it is preferable to achieve a maximum restraint possible on the slant (i.e. displacement of the axial line) of the movable annular member in relation to the cam ring in this cam mechanism. However, between this cam ring and the movable annular member, there is a relationship of fit provided in order to permit a relative movement between the cam groove and the cam pin, with a gap necessarily formed between these two. Thus, it has been found that the conventional cam mechanism for a lens barrel has the disadvantage that the movable annular member is liable to slant in a particular direction in relation to the cam ring because of the presence of such a gap.
With reference to FIG. 1 and FIG. 2, a description will be given below in respect to the direction in which the slant, mentioned above, occurs and the reason for its occurrence in the cam mechanism in the conventional construction.
As shown in FIG. 1, a movable annular member 12 is fitted into the inside of the inner circumference of a cam ring in such a manner as to permit its free rotating motion in the same axial line as that of the rotating motion of the cam ring 11. Also, the cam ring 11 has a cam groove 13 formed in each of two locations opposite to each other in the radius direction in the state of its penetration in the thickness direction, as illustrated in FIG. 2. In the meanwhile, cam pins 14, which are to be set respectively in the cam grooves 13, are set in the surface on the outer circumference of the movable annular member 12.
In this conventional construction, a segment Q, of a line connecting the centers of the pair of cam pins 14, is defined as the axial line of rotation crossing the optical axis O at right angles. It is thereby made possible for the movable annular member 12 to be moved, in its rotation centering around this axial line Q of rotation, in relation to the cam ring 11, in correspondence to a clearance c, delimited by the inner circumferential surface of the cam ring 11 and the outer circumferential surface of the movable annular member 12. In consequence of this construction, it is likely that the movable annular member 12 will have a slant more predominantly in the thickness direction of this clearance c. As compared with this, the slant, which occurs in the front group lens frame (not illustrated in the drawings) as the result of its rotation centering around the axis R crossing the axial line Q of rotation at right angles, will not be so much as the slant caused by its rotation centering around the axial line Q of rotation, because the relationship of fit between the cam groove 13 and the cam pin 14 restrains the slant in this case.
Such a slant of the movable annular member has not caused previous problems in a construction wherein the cam ring 11 and the movable annular member 12 respectively, are made of metal material that delivers a high degree of fitting accuracy. However, there has recently been a tendency to use a variety of resin-made annular members, including the cam ring 11 for a lens barrel, due to a reduction of weight and/or a reduction of costs. Such annular members, made of resin, are inferior in fitting accuracy as compared with annular members made of metal material. Hence, a slant of an annular member like the one described above, has come to present a more notable problem. Thus, it is currently desired to resolve this problem.
Moreover, such a slant of the movable annular member 12 for the cam mechanism is in the worst condition when the cam grooves 13 and the cam pins 14 are respectively provided at an interval of 180.degree., but this may also occur under other conditions.